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1 September 2015

Hydraulic fluids are subject to a wide range of operating conditions that are dependent on the application and the environment where they are used. Some applications are stationary, some are mobile, and among those two broad categories there are a vast number of different applications.

Among those different applications, some operate in relatively mild climates and are protected from the elements, while others operate out in the open and are exposed to varying degrees of temperature change. Equipment that operates outside can experience extreme conditions; through a yearly cycle equipment might experience temperature changes from freezing rain and snow in the winter to searing summer-time heat. Sometimes the temperature cycle in a single day can vary as much as 40 to 50 degrees Fahrenheit. These changes in temperature create challenges for hydraulic systems that primarily result from changes in the viscosity of the oil.

It is important to understand two concepts: Viscosity and Viscosity Index. Let’s start with Viscosity. I’m sure many of you think Viscosity refers to how “thick” the oil is, right? Well, that is really not a good definition. Technically, the term Viscosity refers to a fluid’s resistance to flow. A high-viscosity fluid will resist flowing, while a low-viscosity fluid will flow easily and rapidly.

Viscosity is widely considered the most important characteristic of a lubricant, as building and maintaining a protective film between moving parts depends on having the right viscosity for the application. If the viscosity is either too high or too low, the film will not develop or it will collapse under load, allowing contact between parts. Lubrication engineers consider speed, load and operating temperature when making viscosity recommendations for a given application.

Okay, so what about Viscosity Index? Viscosity Index, or VI, is a numerical value applied to a lubricant based on the degree of change in viscosity relative to a change in temperature. A low VI number indicates a fluid with a high rate of change in viscosity relative to changes in temperature, while a high VI number indicates a low rate of change relative to changes in temperature.

The viscosity of all lubricants changes as the temperature changes. For example, as the temperature goes up, viscosity goes down, and as the temperature goes down, viscosity goes up. However, not all fluids have the same rate of change. A couple of examples may help to illustrate this concept. Think for a moment about a can of shortening you may have in your kitchen cupboard at home. At room temperature it has the consistency of a thick paste and it has a high resistance to flow. Scoop some of it out of the can and put it in a frying pan and it will just sit there and not flow at all. But set the pan on a lit burner and as the shortening heats up, it will melt and flow easily across the bottom of the pan. This is an example of a fluid with a low VI. Now think about synthetic motor oil that flows easily at room temperature. You can take a bottle and put it in the freezer overnight and then take it out in the morning. When you pour it out, it flows just about as easily as it did at room temperature. Any change in viscosity between the two temperatures is hard to detect. This is an example of a fluid with high VI.

High VI fluids that retain their flow characteristics over a wider temperature range offer several advantages for hydraulic systems; this brings us back to the topic of this article. Hydraulic fluids must perform all of the functions of a typical lubricant, i.e. they must lubricate and prevent wear of moving components, cool those components, remove contaminants and provide a seal to prevent the ingression of additional contaminants. In addition, hydraulic fluids must also transmit power to perform work.

Hydraulic systems direct fluid under pressure to operate actuators (cylinders and motors) to move machinery. They operate under varying pressures; fairly low, in the hundreds of pounds per square inch, to over 6,000 psi in some high-pressure systems. The higher the operating pressure, the higher the demand placed on the fluid. High-pressure systems can stress oil with shear forces, which cause an increase in the temperature of the fluid. In a system already under stress because of high ambient temperatures, the added stress from high operating pressures will likely exacerbate the problem.

As mentioned earlier, when the operating temperature increases on a hydraulic oil, the viscosity decreases, and as the viscosity decreases the ability of the oil to maintain a full fluid film between moving parts also decreases. This can result in wear in heavily loaded areas of the system. The reduced viscosity of the oil also promotes seepage or leakage of the fluid past O-rings and seals that have hardened with age and use. These leaks can be external and can result in loss of fluid and potentially environmental damage. They can also be internal and can reduce the efficiency of the system and generate even more heat.

Reduced viscosity can also affect the feel of the system controls, requiring an operator to modify his/her touch on the controls to maintain the same movement of the actuators. In applications that require a fine touch, this can be troublesome; but, in less demanding applications, the operation of the system can still become erratic, which can cause shock loads on the actuators, as well as on bushings, pins and the frame of the machine.

In extreme cold situations the opposite happens. With low VI fluids, the viscosity greatly increases, which can often result in cavitation erosion of the hydraulic pump and sluggish operation of the system. If the temperature drops enough and the viscosity increases too much, the hydraulic system might not even operate.

High VI fluids help reduce or avoid these problems by maintaining a more-consistent viscosity over a greater range of operating temperatures. On cold winter mornings, there is less cavitation erosion damage to the pumps and on hot summer afternoons, there is less wear and tear on system components, less leakage and more consistent feel at the controls. A single fluid can be used year-round without seasonal change-outs and companies with equipment spread around a large geographical area can consolidate to one fluid for all machines.

High VI fluids provide improved film strength under load to prevent metal-to-metal contact and wear of moving parts within the system. High VI fluids also provide improved sealing performance, which can maintain the seal between moving parts and can prevent the ingression of contaminants. These fluids also reduce the amount of leakage past worn or aged seals, which can reduce environmental damage and save the expense of replacing lost fluid.

High VI fluids are also proven to enhance the efficiency of hydraulic systems, providing an improvement in efficiency versus standard, mono-grade hydraulic oil. As a result, less energy is required to produce the same amount of work or increased production can be achieved by using the same amount of energy. In addition, the feel of the system controls remain more stable and consistent, which can reduce shock loading and wear on the equipment.

All of this can reduce the cost to operate and maintain hydraulic systems. In addition to these benefits, high VI hydraulic oils, such as Castrol® Dual Range HV or Castrol® Trans CHT, can also deliver extended drain intervals because they use high-quality base oils and additive packages that are designed to resist oxidation and aging of the fluid.

Please feel free to contact your Castrol sales representative or field engineer for additional details and support.